Many conventional filter assemblies include a hollow, cylindrical filter which comprises a cylindrical, pleated filter medium sandwiched between and bonded to impervious end caps. At least one of the end caps has a central aperture through which a fluid line communicates with the interior of the filter. Fluid to be filtered is directed through the filter medium from the outside to the inside and the filtered fluid is then removed from the interior of the filter via the fluid line.
As the fluid flows outside-in through the filter, contaminants, such as particulates, accumulate on the outside surface of the filter medium. Once a sufficient accumulation of contaminants has caked onto the outside surface of the filter medium, flow through the filter is significantly impeded. At this point, the filter should be removed from the filter assembly and replaced with a new filter. Unfortunately, with the accumulation of contaminants caked on the outside of the filter medium, removing and replacing the filter can result in the release of some of the accumulated contaminants. These contaminants can then re-enter the system fluid and cause severe wear or other related problems.
A filter assembly having a filter with one closed end cap allows the fluid to be filtered to be directed from the inside-out and significantly diminishes this problem. The contaminants accumulate on the inside surface of the filter medium and therefore are contained within the filter by the medium and closed end cap. Consequently, the filter may be removed and replaced conveniently without release of the accumulated contaminants trapped inside the filter.
Unfortunately, conventional filter assemblies designed for inside-out have several additional drawbacks. For example, in order to force fluid through the filter, the pressure of the fluid inside the filter must be greater than the pressure of the fluid outside the filter. This difference in pressure forces the end caps away from the filter medium. If the force becomes great enough, the bond between the end caps and the filter medium can rupture, causing contaminated fluid to bypass the filter medium. Because of this force, conventional filter assemblies frequently use two open end caps with compression seals and/or an auxiliary spring force to help keep the filter in compression to prevent rupture of the end cap bonds. However, the open end caps will not contain the accumulated contaminant on the inside of the filter.
Inside-out flow is also more likely to stretch and misshape the pleats of the filter medium than outside-in flow. Misshaping the element modifies its porous characteristics, making it useless long before rupture occurs. Further, both of these problems, i.e, rupture of the end cap bonds and misshaping of the element, are heightened by flow disturbances, such as flow surges, which may typically be encountered by the filter assembly.